As described in U.S. Pat. No. 4,074,208, issued to Michael E. Mack, et al on Feb. 14, 1978 and assigned to the assignee hereof, flashlamps have been used extensively as a source of excitation radiation for energizing a laser medium to a lasing condition. For this purpose, the radiation in the flashlamp discharge arc or high energy plasma is typically focused by lenses or mirrors into the laser medium. Laser efficiency depends in part on the degree to which the discharge can be limited in diameter so that the image focused to the center of the laser medium is likewise limited, thereby to couple a maximum amount of pumping energy to the desired lasing zone.
In addition to the requirement for a dimensionally narrowed discharge, in flowing dye lasers the flashlamp must be capable of being rapidly pulsed so as to permit the attaining of maximum laser output. In flowing dye lasers the dye is rapidly replenished, and this permits the application of pumping pulses at a high repetition rate to achieve maximum energy output.
Rapidly pulsed dye lasers are used extensively in the field of isotope separation especially as it relates to separating U-235 from U-238. Such a system is described in U.S. Pat. No. 3,772,519 issued to R. H. Levy, et al for a Method and Apparatus for the Separation of Isotopes and is assigned to the assignee hereof. The efficiency of such a laser isotope separation system depends heavily on the amount of ionizing radiation which can be pumped into the reaction region for the process.
One of the major advances with present dye laser systems has been in the area of increasing the average power capability of the laser through increasing the flashlamp diameter. However, increasing the diameter of the flashlamp decreases the watts/cm.sup.2 through the flashlamp envelope. To make up for this decrease, either an increased repetition rate may be employed, or the energy per pulse can be increased, both of which having thus far proved difficult to achieve for the following reasons:
Present flashlamps used for short pulse excitation of dye lasers typically utilize Xenon. The Xenon flashlamp provides not only green XeII line radiation where desired, but also a continuum of radiation. The XeII line of the Xenon flashlamp is useful because of its short duration which makes possible the production of ultra short flashlamp pulses. However, while the XeII line radiation is short in duration, the continuum radiation can last for many microseconds, and this precludes increasing repetition rates. Moreover, the existence of continuum radiation is undesirable because much of it represents radiation ineffective to excite useful lasing states in the dye laser and thus creates heat.
Additionally, when Xenon flashlamps are operated at high pulse repetition frequencies, typically 150-200 pulses per second, the arc becomes more diffuse resulting in a loss of the XeII line spectra, a reduction in peak excitation rates, and an increase in pulse duration. The diffuse arc which results at high pulse repetition frequencies, when focused from the flashlamp into the laser medium produces a larger focused image within the laser medium. This is undesirable because it results in a lower concentration of excitation energy in the dye laser medium.
As illustrated in the aforementioned patent and in U.S. Pat. No. 3,967,212 issued to Daniel J. Dere, et al on June 29, 1976, and U.S. Pat. No. 3,842,284 issued to Berta, et al on Oct. 15, 1974, a so-called "simmer" current is often used. According to this technique, a continuous current bias is provided to permit a rapid turn on and turn off of the flashlamp with less voltage swing. When utilizing a simmer current in a large diameter tube, the arc will typically attach itself in an unstable manner to the tube wall.
By way of further background, the utilization of additional flashlamp energizing pulses is described in U.S. Pat. No. 4,004,248 issued to Alexander Muller, et al on Jan. 18, 1977, and in U.S. Pat. No. 4,088,965, issued to James B. Lauderslager, et al, in which a helium and nitrogen mixture may be made to lase at a lower pressure through the utilization of a pre-ionization pulse.